The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for reducing peak power requirement and/or power deposition per pulse during a substantially simultaneous multi-slice acquisition.
Since its initial application, scan time for volume coverage with echo-planar imaging (“EPI”) or spiral-type MRI data acquisitions has not substantially decreased. Nearly all the successful efforts to shorten EPI acquisition times have targeted reducing the number of refocused echoes needed for spatial encoding to form an image, such as by means of partial Fourier imaging, parallel imaging, or sparse data sampling techniques. Although these approaches decrease scan time for spatial encoding of a single slice in EPI, they do not necessarily reduce the time required for image acquisitions by a significant amount. This lack in scan time reduction is because if multiple slices are employed to cover the volume—as is most often the case—the time of volume coverage is equal to the product of the number of slices needed to cover the volume and the acquisition period of each slice. The image acquisition period for each slice remains significant even when spatial encoding times are shortened by techniques like parallel imaging. This lack of scan time reduction is especially true when a physiological contrast preparation period (e.g. for imaging neuronal activity or water diffusion) precedes the spatial encoding period for each slice; the former can equal or exceed the latter in rapid imaging sequences such as EPI because it must be repeated for each slice. The problem is the same for fast acquisition techniques such as turbo-spin echo (“TSE”) or fast spin echo (“FSE”); namely, multiple refocused echoes are formed using 180-degree pulses, as opposed to gradient reversal in EPI, to cover multiple k-space lines.
The foregoing problem also exists for normal imaging where one k-space line is collected after each radio frequency (“RF”) excitation pulse, whether sampling k-space along a radial trajectory or along a rectilinear trajectory, as in a gradient-recalled echo (“GRE”) pulse sequence or a spin-echo (“SE”) pulse sequence. In fact, the problem is exacerbated in these cases because each slice is collected not in a single application of the RF pulse (as is done in single shot EPI) or in few shots of the RF pulse (as in segmented EPI); rather, the RF pulse is applied for each k-space line, one k-space line at a time, requiring numerous application of the RF pulse to generate a single image.
Recently, significant shortening of the scan time required for volume coverage has been demonstrated by “slice multiplexing,” in which multiple image slice locations are excited and acquired simultaneously using a multiband (“MB”) radio frequency (“RF”) pulse, a technique commonly referred to as multiband imaging.
In MB imaging there are number of limitations. One limitation is the RF power required to produce the multiband RF pulses. This limitation arises because an MB pulse with an MB factor—the number of simultaneously excited slices—greater than one requires substantial increased power relative to when the same pulse form is employed to excite a single slice versus multiple slices.
The RF pulse with an MB factor greater than one can be viewed as the sum of all the individual RF pulses that excite one slice, which results in a pulse that increases the voltage applied to the coil linearly with the number of pulses; the power required for one application of the pulse then goes as the square of the voltage integrated over the pulse duration. Maximal MB factors that can be practically achieved due to power limitations may arise because of the maximum voltage, current, or power available in the MRI system, which may in turn be imposed by the output capacity of RF amplifiers used in the system or the voltage, current, or power tolerance of the RF front end (i.e., the components involved in the delivery of the RF pulse to the RF coil used to transmit RF to the sample being studied), including the RF coil itself. Alternatively, the achievable MB factor, may be limited because of power deposition into the subject. In human imaging, power deposition averaged over a given period of time cannot exceed limits dictated by regulatory agencies such as the FDA.
SAR, which is a measure of the rate at which energy is absorbed by the body when exposed to an RF electromagnetic field and is measured in watts per kilogram of tissue (“W/kg”), is a concern when conducting MRI experiments on human subjects. As noted above, SAR is especially a concern during the simultaneous excitation of multiple slice locations. This is because when multiple RF pulses are simultaneously employed, the local electric fields generated by each RF pulse undergo local superposition and local extremes in electric field magnitude may arise, leading to spikes in local and global SAR that are of concern to regulatory bodies in both the United States and Europe. For a discussion of these regulatory concerns in the United States, see, for example, Center for Devices and Radiologic Health “Guidance for the Submission of Premarket Notifications for Magnetic Resonance Diagnostic Devices,” Rockville, Md.: Food and Drug Administration; 1998, and in Europe, see, for example, International Electrotechnical Commission, “International Standard, Medical Equipment-Part 2: Particular Requirements for the Safety of Magnetic Resonance Equipment for Medical Diagnosis, 2nd Revision,” Geneva: International Electrotechnical Commission; 2002.
The need to stay below safe SAR limits often requires unfavorable tradeoffs in acquisition parameters such as reduced bandwidth of the RF pulse, reduced flip angle of the RF pulse, or increased repetition time between consecutive applications of RF pulses needed to complete the acquisition of a single volume-of-interest (i.e., inter-pulse TR) or between consecutive acquisitions of the volume-of-interest when the same volume is to be imaged repeatedly (volume TR). SAR becomes especially problematic at field strengths of 3T and higher, where the power deposited for a given flip angle increases approximately quadratically with magnetic field magnitude; thus, an increase of as much as four-fold as compared to 1.5T applications can be present.
It would therefore be desirable to provide a method manipulating spins at multiple slice locations with a reduction in peak voltage, peak power, and/or power deposition, which may be measured as SAR. Examples of spin manipulation include excitation, inversion, and refocusing. Such a method would broaden the applicability of multiband RF pulses to imaging pulse sequences other than historically low peak RF pulse power and low SAR sequences, and would allow for improving other imaging parameters, such as TR and volume coverage.